1. Field of the Invention
The present invention relates to image pick-up apparatus such as radiographic apparatus and the like and, more particularly, to image pick-up apparatus and an image pick-up system provided with a reference supply circuit for supplying a reference voltage for driving of an area sensor.
2. Related Background Art
A conventional example of image pick-up apparatus will be described below with the drawings. FIG. 7 is an equivalent circuit diagram of the conventional example and FIGS. 8A and 8B are diagrams showing its drive timing and output. As shown in FIG. 7, the conventional apparatus is provided with an area sensor in which pixels p1.1-3.3, each consisting of a photodiode as a detecting element and a TFT as a switching device for the photodiode, are arranged in a two-dimensional array and which is driven by matrix driving; a gate drive device 2 for driving the TFTs of the area sensor; a read device 1 to which signals outputted by the driven TFTs are transferred; and a reference supply circuit 4 for supplying reference voltages for driving or the like of the area sensor and other devices.
A gate electrode of the TFT in each pixel is coupled to a common gate line Vg1-Vg3, and the common gate lines are coupled to the gate drive device 2 comprised of an unrepresented shift register and the like. A source electrode of each TFT is coupled to a common data line sig1-sig3 to be coupled to the read device 1 comprised of amplifiers, sample hold circuits, an analog multiplexer, and so on.
In the present conventional example the reference supply circuit 4 supplies the following reference voltages.                Sensor bias voltage (Vs)        Sample hold reference voltage (VREF1)        Amp reset voltage (VREF2)        TFT on voltage (Von)        TFT off voltage (Voff)        
Among these, the sensor bias voltage (Vs) is supplied directly from the reference supply circuit 4 to the area sensor. The sample hold reference voltage (VREF1) is supplied to the sample hold circuits of the read device 1, and the amp reset voltage (VREF2) to the amplifiers of the read device 1. The TFT on voltage (Von) and off voltage (Voff) are supplied through the gate drive device 2 to the area sensor.
The operation will be described below with the timing chart of FIG. 8A. A reading operation for one line will be described. In FIG. 8A, RES represents a reset signal to the amplifiers of the read device 1, Vg1-3 timing signals of a gate pulse to the respective common gate lines, SMPL a sampling timing signal to the sample hold circuits of the read device 1, and CLK an analog synchronization signal from the analog multiplexer of the read device 1.
The amplifiers are first reset, the TFTs are then turned on (Vg1), signal charges in the photodiodes of p1.1-p3.1 are transferred thereby to the amplifiers of the read device, the sample hold circuits (SMPL) sample and hold the charges, and the analog multiplexer provides an analog output thereof in synchronism with CLK. FIG. 8B is a diagram showing the output.
As described above, the conventional image pick-up apparatus is provided with the reference supply circuit to supply the reference voltages to the sensor, the gate drive device, the read device, and so on. FIG. 9 is a circuit diagram showing an example of the reference supply circuit in the conventional image pick-up apparatus. A device for regulating each reference voltage of the sensor bias voltage Vs, the TFT off voltage Voff, the TFT on voltage Von, and the amp reset voltage VREF2 is comprised of a 3-terminal regulator (e.g., TL317 available from Texas Instruments).
However, the supply circuit such as series regulator IC or the like has a random noise component due to flicker noise and thermal noise of semiconductors. For example, when the reference supply circuit is constructed of AD780 being the series regulator IC available from Analog Devices, the random noise component is approximately 100 nV/√{square root over (Hz)}.
In the conventional image pick-up apparatus, this random noise component of the reference supply circuit sometimes caused line noise in an image. With variation in the amp reset voltage VREF2, the sample hold reference voltage VREF1, the TFT off voltage Voff, and the sensor bias voltage Vs in FIG. 7, there occurred the line noise as shown in the “actual analog signal” in FIG. 8B in certain cases. The line noise is given by the difference between the “ideal analog output” and the “actual analog signal” in FIG. 8B. Such line noise appeared in a stripe pattern in an image reproduced from the signal read by the area sensor, and degraded the quality of the image in some cases.
Particularly, high frequency components out of noise components of the reference voltages become beatlike line noise through sampling with the SMPL signal and are considered to have great effect on degradation of image quality. The occurrence of these line noise sometimes posed a problem in particular in the case where it was necessary to acquire the image data with a high degree of accuracy, e.g., in the case of X-ray image pick-up apparatus and the like.
The present invention has been accomplished in view of the above point and an object of the present invention is to provide image pick-up apparatus and an image pick-up system constructed to prevent the occurrence of the line noise due to the random noise component originating in the reference supply circuit for supplying the reference voltages